I definitely under estimated what it would be like to use them and I am fascinated reading other peoples blogs and how much they use them. I will have to build up to that over time, but this week plan on sticking to the 4 x 30 minute sessions on alternate cycling days.
My quads get tired from the pushing action, but you are forced more than usual to actually "pull" so the hamstrings get a descent work out, like they would if your were running. It is muscular fatigue more than anything the I feel.
In my situation, yes, it would be fantastic to see improvements in my cycling, but my sole purpose is to assist my running, or the lack off, due to injury. I am using the power cranks on my alternate run days to continue to work those muscle with out the pounding of road running.
Tomorrow I will use them again for session number 3 and hopefully again see some slight improvements in my efficiency more than anything else, which in the end will allow me to use them for longer.